Gas turbine engine heat management system

ABSTRACT

A heat management system of a gas turbine engine for cooling oil and heating fuel, includes an oil circuit having parallel connected first and second branches. The first branch includes a fuel/oil heat exchanger and a first fixed restrictor in series and the second branch includes an air cooled oil cooler and a second fixed restrictor. The first and second fixed restrictors limit respective oil flows through the first and second branch differently, in response to viscosity changes of the oil caused by temperature changes of the oil during engine operation to modify oil distribution between the first and second branches.

TECHNICAL FIELD

The application relates generally to gas turbine engines, and moreparticularly, to a heat management system of a gas turbine engine forcooling oil and heating fuel.

BACKGROUND OF THE ART

A heat management system of a gas turbine engine conventionally includesa fuel/oil heat exchanger (FOHE) to transfer heat from the hot oil tothe cold fuel in order to heat the cold fuel to a desired temperature.An air cooled oil cooler (ACOC) is also conventionally provided in theheat management system to further cool the hot oil to a lowertemperature in order to be recycled in an oil circuit of the engine.ACOCs and FOHEs are conventionally connected in series and a thermalstatic valve is also provided to allow an oil flow to selectively bypassthe ACOC, for example in cold oil conditions. However, the conventionalthermal static valves generally have very low reliability, which drivesup maintenance costs.

Accordingly, there is a need to provide an improved system for gasturbine engines.

SUMMARY

In one aspect, there is provided a heat management system of a gasturbine engine for cooling oil and heating fuel, the system comprisingan oil circuit having connected first and second branches in a parallelconfiguration, the first branch including a fuel/oil heat exchanger fortransferring heat from an oil flow through the first branch, to a fuelflow and a first fixed restrictor for restricting the oil flow throughthe first branch, the second branch including an air cooled oil coolerfor air cooling an oil flow through the second branch and a second fixedrestrictor for restricting the oil flow through the second branch, thefirst and second fixed restrictors having respective fixed passagegeometries, the passage geometry of the second fixed restrictor having atotal flow contact area greater than a total flow contact area of thefirst fixed restrictor such that in response to an oil viscosityincrease, the second fixed restrictor provides a larger flow resistanceincrease than a flow resistance increase provided by the first fixedrestrictor.

In another aspect, there is provided a heat management system of a gasturbine engine for cooling oil and heating fuel, the system comprisingan oil circuit having connected first and second branches in a parallelconfiguration, the first branch including a fuel/oil heat exchanger fortransferring heat from an oil flow through the first branch, to a fuelflow and a first fixed restrictor disposed downstream of the fuel/oilheat exchanger for restricting the oil flow through the first branch,the second branch including an air cooled oil cooler for air cooling anoil flow through the second branch and a second fixed restrictordisposed downstream of the air cooled oil cooler for restricting the oilflow through the second branch, wherein the first fixed restrictorincludes a diaphragm having a flow orifice and the second fixedrestrictor includes a plurality of holes extending through a body, theholes being small and long with respect to the flow orifice of the firstfixed restrictor.

In a further aspect, there is provided a method of managing oil coolingand fuel heating in a gas turbine engine, the method comprising a)distributing oil from a pumped oil supply into first and second oilflows, the second oil flow being parallel to the first oil flow; b)transferring heat from the first oil flow to a fuel flow; c) usingambient air to cool the second oil flow; and d) using a combination oftwo fixed restrictors to limit the respective first and second oil flowsdifferently, in response to viscosity changes of the oil caused bytemperature changes of the oil during engine operation.

Further details of these and other aspects of the described subjectmatter will be apparent from the detailed description and drawingsincluded below.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying drawings depicting aspects ofthe described subject matter, in which:

FIG. 1 is a schematic cross-sectional view of an aircraft turbofan gasturbine engine as an exemplary application of the described subjectmatter;

FIG. 2 is a schematic illustration of a heat management system accordingto one embodiment;

FIG. 3 is a partial cross-sectional view of a fuel/oil heat exchanger(FOHE) restrictor showing a diaphragm having a flow orifice according toone embodiment;

FIG. 4 is a partial cross-sectional view of the FOHE restrictor showinga diaphragm having a sharp edged flow orifice according to anotherembodiment; and

FIG. 5 is a cross-sectional view of an air cooled oil cooler (ACOC)restrictor and a surface of a body with a plurality of small holesextending through the body.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION

Referring to FIG. 1, an aircraft turbofan gas turbine engine includes ahousing or nacelle 10, a core casing 13, a low pressure spool assembly(not numbered) which includes a fan assembly 14, a low pressurecompressor assembly 16 and a low pressure turbine assembly 18 connectedby a shaft 12, and a high pressure spool assembly (not numbered) whichincludes a high pressure compressor assembly 22 and a high pressureturbine assembly 24 connected by a turbine shaft 20. The core casing 13surrounds the low and high pressure spool assemblies to define a mainflow path or gas path (not numbered) therethrough. In the main flow paththere is provided a combustion gas generator assembly 26 to generatecombustion gases for powering the high and low pressure turbineassemblies 24, 18. There is also provided a fuel supply system 28 forsupplying fuel to the combustion gas generator assembly 26. There isfurther provided a heat management system 30 for cooling hot oilcirculated in an oil system (not shown) of the engine and for heatingthe fuel prior to being delivered for combustion. The heat managementsystem 30 schematically illustrated in FIG. 1, does not represent aspecific structure and location in the engine.

Referring to FIGS. 1 and 2, the heat management system 30 includes anoil circuit illustrated as a block defined by broken lines 32 in FIG. 2.In one embodiment, the oil circuit 32 may include a first branch 34 anda second branch 36 connected in a parallel configuration. The firstbranch 34 may include a fuel/oil heat exchanger (FOHE) 38 and an FOHErestrictor 40 in series. For example the FOHE restrictor 40 may bedisposed downstream of the FOHE 38. Optionally, a pressure relief valvesuch as a check-valve 42 may also be provided in the first branch 34,for example disposed downstream of the FOHE 38 and parallel to the FOHErestrictor 40.

In one embodiment, the second branch 36 may include an air cooled oilcooler (ACOC) 44 and an ACOC restrictor 46 in series. For example theACOC restrictor 46 may be disposed downstream of the ACOC 44.

The heat management system 30 may further include an oil pump 48 and oilfilter 50 which are disposed upstream of and connected to the oilcircuit 32 such that oil pump 48 pumps oil from an oil tank 52 whichcontains relatively hot oil collected from, for example bearing chambers(not shown) of the engine during engine operation, to the oil circuit32, splitting the oil into first and second oil flows passing throughthe respective parallel first and second branches 34, 36. The first andsecond oil flows from the first and second branches 34, 36 are combinedand directed into an engine oil manifold 54 which is disposed downstreamof and connected to the oil circuit 32. The engine oil manifold 54distributes the oil to various locations of the engine to lubricate andcool for example bearings and gears of the engine.

The FOHE 38 includes oil passages (not numbered) forming part of thefirst branch 34 of the oil circuit and fuel passages (not shown) whichare connected in the fuel system. Therefore, cold fuel from a fuelsupply 56 can be directed through the FOHE 38 and can be thus heated bythe first flow of the hot oil passing through the first branch 34 of theoil circuit 32. The heated fuel from the FOHE 38 may be directed forexample, through a filter 58 to an engine fuel control unit FCU 60 whichcontrols fuel delivery at a required rate to the combustion gasgenerator assembly 26.

The ACOC 44 includes oil passages (not shown) exposed to for example, anambient air stream 62 passing through a bypass duct (not numbered) ofthe engine. Therefore, the second oil flow passing through the secondbranch 36 of the oil circuit 32 is cooled by the relatively cool ambientair stream 62. The oil in the engine oil manifold 54 is a mixture of thefirst oil flow which passes through the first branch 34 of the oilcircuit 32 and is cooled in the FOHE 38 by cold fuel, and the second oilflow which passes through the second branch 36 of the oil circuit 32 andis cooled in the ACOC 44 by the cold ambient air stream 62. Therefore,the oil in the engine oil manifold 54 is cooler than the oil in the oiltank 52.

During engine operation the fuel flow required for combustion and thetemperatures of the hot oil flowing through the oil circuit may vary andtherefore the heat exchange performed in the FOHE 38 and ACOC 44 must becontrolled accordingly. A thermal static valve (also known as a thermalvalve) is conventionally used in an engine heat management system forthis purpose, as discussed above in the Background of the Art. In theheat management system 30, the thermal valve may be eliminated. The oilflow split between the FOHE 38 and the ACOC 44 is controlled by acombination of the FOHE restrictor 40 and the ACOC restrictor 46, whichlimits the first and second oil flows through the first and secondbranches 34, 36 differently, in response to viscosity changes of the oilcaused by temperature changes of the oil during engine operation.

According to one embodiment, both the FOHE restrictor 40 and ACOCrestrictor 46 are fixed restrictors which, however have different fixedpassage geometries. The passage geometry of the ACOC restrictor 46 has atotal flow contact area greater than a total flow contact area of theFOHE restrictor 40 such that in response to an oil viscosity increase,the ACOC restrictor 46 provides a larger flow resistance increase than aflow resistance increase provided by the FOHE restrictor 40, in order tochange oil flow distribution between the first and second branches 34,36 of the oil circuit 32.

In one embodiment illustrated in FIGS. 2 and 3, the FOHE restrictor 40may be a calibrated diaphragm restrictor which, for example, includes aflow chamber 64 and a diaphragm 66 disposed within the flow chamber 64as a partition. The diaphragm 66 defines an orifice 68 extendingtherethrough to allow the first oil flow in the first branch 34 of theoil circuit 32 to pass through the chamber 64. The orifice 68 iscalibrated to limit the first oil flow passing through the FOHE 38 inorder to prevent the fuel flow from being overheated. The diameter ofthe chamber 64 is much larger than the diameter of the orifice 68.Alternatively, the diaphragm 66 according to one embodiment, may definea flow orifice 70 having a sharp annular edge 72 with an edge tip angleA smaller than 90 degrees, as illustrated in FIG. 4.

The ACOC restrictor 46 on the other hand, according to one embodimentillustrated in FIGS. 2 and 5, may define a plurality of small and longholes 74 with respect to the flow orifice 68 of the FOHE restrictor 40.The small and long holes 74 extend through a body 76 which is disposedin a flow chamber 78 of the ACOC restrictor 46. Each of the small andlong holes 74 has a diameter smaller than an axial length of the hole74.

The performance of the ACOC 44 may be chosen to provide adequate oilcooling when the engine is operated at high altitudes and theperformance of the FOHE 38 is chosen to provide adequate heat transferfrom oil into fuel in cold conditions. The ACOC restrictor 46 may becalibrated to allow the second oil flow in the second branch 36 of theoil circuit 32 to flow almost unrestrictedly through the ACOC restrictor46 when the oil is very hot and thus the oil viscosity is very low. TheFOHE restrictor 40 may be calibrated in order to limit the first oilflow through the first brand 34 of the oil circuit to a rate that avoidsoverheating the fuel flow when the engine is operated at high altitudesand in order to send the rest of the oil to the second oil flow passingthrough the second branch 36 of the oil circuit. For a given oiltemperature, the oil flow split between the first and second branches34, 36 may remain at approximately the same at any altitude. At loweraltitudes the performance of the ACOC 44 improves significantly due tothe increased air density and, consequently, increased air mass flowingthrough the ACOC 44. The increased performance of the ACOC 44 may matchthe increased engine heat rejection at lower altitudes, which engineheat rejection is also proportional to the increased pressure in the gaspath of the engine. There may also be a marginal increase in the heattransferred from oil into fuel in the FOHE 38 due to the increased fuelflow at the lower altitudes.

At a lower ambient temperature the second oil flow in the second branch36 exiting from the ACOC 44 is cooler. The ACOC restrictor 46 thereforeoffers increased flow resistance to the second oil flow in the secondbranch 36 due to increased oil viscosity when the second oil flow in thesecond branch 36 is cooler. This results in a reduction of the secondoil flow through the second branch 36 and a corresponding increase ofthe first oil flow in the first branch 34 of the oil circuit. Meanwhile,the flow resistance provided by the FOHE restrictor 40 is substantiallyindependent from the temperatures of the first oil flow flowing throughthe first branch 34 because the flow resistance determined by the fixedgeometry of the orifice 68 or 70 of the FOHE restrictor 40 is notsignificantly affected by oil viscosity changes with respect to the ACOCrestrictor 46. The second oil flow reduction in the second branch 36 ofthe oil circuit when the ambient air temperatures are low, determinesfurther oil cooling in the ACOC 44 (because not only the ambient airstream is cooler but also less oil is concurrently being cooled by thecooler air stream). At lower ambient temperatures, the temperature ofthe second oil flow exiting from the ACOC 44 reaches the ambient airtemperature and the second oil flow in the second branch 36 may bereduced to minimum while the first oil flow in the first branch 34 inthe oil circuit 32 is increased to maximum. The increased first oil flowat low ambient air temperatures may ensure that an optimum amount ofheat is transferred from the engine oil system to the engine fuelsystem.

The check-valve 42 (for pressure relief) in the first branch 34 isnormally closed and opens only when the oil pressure build-up in thefirst and second branches 34, 36 in the oil circuit 32, reaches apredetermined level, in order to prevent the FOHE 38 and ACOC 44 frombeing damaged.

Alternatively, the FOHE restrictor 40 and the check-valve 42 may becombined in one unit, such as a pressure relief valve with calibratedflow leakage.

The heat management system 30 may eliminate or reduce the requirementfor thermal valves, or for commanded/actuated control valves. Thecombination of the fixed restrictors 40, 46, with an optional pressurerelief valve, is simpler, cheaper and may have significantly betterreliability than thermal valves or commanded/actuated control valves,for controlling oil flow distribution between the ACOC and the FOHE 38,44. The heat management system 30 allows a relatively simple systemarchitecture and optimum component sizing. It should also be noted thatsince oil viscosity changes exponentially with respect to oiltemperature, the thermal control offered by the heat management system30 may therefore be quite accurate and without hysteresis.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departure from the scope of the described subjectmatter. For example, the fixed passage geometries of the respective FOHErestrictor 40 and ACOC restrictor 46 may be any suitable and may thusvary from the structures in the described embodiments. The oil circuit32 of the heat management system as described above, may bealternatively positioned to receive used hot oil from bearing chambersand to discharge cooled oil to an oil tank of the engine. Still othermodifications which fall within the scope of the described subjectmatter will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. A heat management system of a gas turbine engine for cooling oil andheating fuel, the system comprising an oil circuit having first andsecond branches connected in parallel, the first branch including afuel/oil heat exchanger for transferring heat from an oil flow throughthe first branch to a fuel flow, and a first fixed restrictor forrestricting the oil flow through the first branch, the second branchincluding an air cooled oil cooler for air cooling an oil flow throughthe second branch and a second fixed restrictor for restricting the oilflow through the second branch, the first and second fixed restrictorshaving respective fixed passage geometries, the passage geometry of thesecond fixed restrictor having a total flow contact area greater than atotal flow contact area of the first fixed restrictor such that inresponse to an oil viscosity increase, the second fixed restrictorprovides a larger flow resistance increase than a flow resistanceincrease provided by the first fixed restrictor to modify oil flowdistribution between the first and second branches.
 2. The system asdefined in claim 1 wherein the first fixed restrictor is disposeddownstream of the fuel/oil heat exchanger and the second fixedrestrictor is disposed downstream of the air cooled oil cooler.
 3. Thesystem as defined in claim 2 wherein the first branch further comprisesa pressure relief valve disposed downstream of the fuel/oil heatexchanger and in parallel connection with the first fixed restrictor. 4.The system as defined in claim 1 comprising an oil pump disposedupstream of the oil circuit.
 5. The system as defined in claim 1comprising an engine oil manifold disposed downstream of the oilcircuit.
 6. The system as defined in claim 1 wherein the fuel flow isdirected from an engine fuel supply to an engine fuel control unit. 7.The system as defined in claim 1 wherein the first fixed restrictor is acalibrated diaphragm restrictor.
 8. The system as defined in claim 1wherein the fixed geometry of the second fixed restrictor is defined bya plurality of holes extending through a body.
 9. A heat managementsystem of a gas turbine engine for cooling oil and heating fuel, thesystem comprising an oil circuit having connected first and secondbranches in a parallel configuration, the first branch including afuel/oil heat exchanger for transferring heat from an oil flow throughthe first branch, to a fuel flow and a first fixed restrictor disposeddownstream of the fuel/oil heat exchanger for restricting the oil flowthrough the first branch, the second branch including an air cooled oilcooler for air cooling an oil flow through the second branch and asecond fixed restrictor disposed downstream of the air cooled oil coolerfor restricting the oil flow through the second branch, wherein thefirst fixed restrictor includes a diaphragm having a flow orifice andthe second fixed restrictor includes a plurality of holes extendingthrough a body, the holes being small and long with respect to the floworifice of the first fixed restrictor.
 10. The system as defined inclaim 9 wherein the first branch comprises a pressure relief valvedisposed downstream of the fuel/oil heat exchanger and in parallelconnection with the first fixed restrictor.
 11. The system as defined inclaim 9 wherein the flow orifice of the first fixed restrictor has asharp annular edge having an edge tip angle of less than 90 degrees. 12.The system as defined in claim 9 wherein each of the holes of the secondfixed restrictor has a diameter smaller than an axial length of thehole.
 13. A method of managing oil cooling and fuel heating in a gasturbine engine, the method comprising: a) distributing oil from a pumpedoil supply into first and second oil flows, the second oil flow beingparallel to the first oil flow; b) transferring heat from the first oilflow to a fuel flow; c) using ambient air to cool the second oil flow;and d) using a combination of two fixed restrictors to limit therespective first and second oil flows differently, in response toviscosity changes of the oil caused by temperature changes of the oilduring engine operation.
 14. The method as defined in claim 13 whereinstep (d) is performed by directing the first oil flow through a firstfixed restrictor having a first fixed passage geometry with a first flowresistance, the first flow resistance being substantially independent oftemperatures of the first oil flow, and directing the second oil flowthrough a second fixed restrictor having a second fixed passage geometrywith a second flow resistance, the second flow resistance varying inresponse to temperature changes of the second oil flow to modify oildistribution between the first and second oil flows in response totemperature changes of the oil.
 15. The method as defined in claim 14comprising calibrating the first fixed restrictor to allow a maximumflow rate, thereby preventing the fuel flow from being overheated. 16.The method as defined in claim 14 comprising directing the first oilflow to bypass the first fixed restrictor when an oil pressure build-upof the first and second oil flows reaches a predetermined level.
 17. Themethod as defined in claim 13 comprising combining the first and secondoil flows after step (d) and directing the combined first and second oilflows into an engine oil manifold.